Red Blood CellsEdit

Red blood cells (RBCs), or red blood cell, are the most abundant cellular component of blood in mammals and a central pillar of circulatory physiology. Their primary role is to ferry oxygen from the lungs to tissues throughout the body and to return carbon dioxide, a waste product of metabolism, to the lungs for exhalation. The efficiency of this transport hinges on a protein called hemoglobin and on the distinctive structural design of the cells themselves.

In humans, mature RBCs are small, disc-shaped, and biconcave, about 6–8 micrometers in diameter. This shape provides a large surface area relative to volume, which accelerates gas diffusion. Mature RBCs are anucleate and lack mitochondria, kidneys of energy, and other organelles, which frees space for hemoglobin and reduces cellular maintenance needs. The hematocrit—the proportion of blood volume occupied by RBCs—typically sits in the mid-30s to mid-40s in healthy adults, with modest variation by sex, altitude, and health status. hemoglobin and the iron-containing heme groups within it enable oxygen binding; the same molecule also participates in carbon dioxide transport in a form that shifts with tissue needs, such as Bohr effect-driven changes in oxygen affinity.

Structure and function

Hemoglobin is a tetrameric protein with iron-containing heme groups that reversibly bind O2 and CO2. Each RBC can carry a large load of oxygen when hemoglobin is saturated, and it also acts as a buffer for blood pH during gas exchange. See hemoglobin for the molecular architecture and the chemistry of gas binding.
The RBC membrane is a composite of lipids and a cytoskeletal scaffold (including proteins such as spectrin and ankyrin) that preserves the biconcave shape during circulation and endows the cell with durability under shear stress. Disruptions in membrane integrity or cytoskeletal links can lead to fragility disorders like hereditary spherocytosis.
Metabolism in RBCs is dominated by glycolysis because mature cells lack mitochondria; this anaerobic energy production supports maintenance of cell shape and ion gradients. The glycolytic pathway also feeds into the production of 2,3-bisphosphoglycerate (2,3-BPG), a molecule that modulates hemoglobin’s oxygen affinity to match tissue oxygen demands. See glycolysis and 2,3-BPG for more.
Oxygen transport and gas exchange are complemented by carbonic anhydrase, which rapidly interconverts carbon dioxide and bicarbonate, helping to transport CO2 in blood to the lungs. See carbonic anhydrase.

Development and lifespan

RBCs arise from hematopoietic stem cells in the bone marrow through a process called erythropoiesis. The maturation sequence includes stages such as proerythroblast, erythroblast, normoblast, and finally mature reticulocyte, which is released into circulation and rapidly becomes an RBC. The hormone erythropoietin is a key regulator, increasing RBC production in response to hypoxic stress or blood loss.

Once mature, RBCs circulate for about 120 days in humans before being cleared by the spleen and liver, where macrophages remove aged cells and recycle iron from heme. Because mature RBCs lack nuclei and mitochondria, their lifespan and breakdown are tightly linked to membrane integrity and metabolic capacity rather than cellular repair. See reticulocyte for the immature form released during erythropoiesis, and spleen for the site of much of RBC clearance.

Oxygen transport and gas exchange

Gas transport is the core function of RBCs. Oxygen binds to iron in the heme groups of hemoglobin within the lungs, travels through the bloodstream, and is released where tissues require it. Carbon dioxide, a metabolic waste product, is carried back to the lungs primarily as bicarbonate in plasma, with a portion bound to hemoglobin in a form known as carbaminohemoglobin. As tissue pH and CO2 levels change, the oxygen affinity of hemoglobin shifts (the Bohr effect), facilitating release of O2 where it is most needed and pickup of CO2 where it is abundant. Hemoglobin’s ability to efficiently bind and release gases under varying conditions is central to cellular respiration and energy production across organs. See oxygen, carbon dioxide, and hemoglobin.

Clinical relevance and variation

RBCs are highly responsive to health status, environmental factors, and genetic variation. Conditions that affect RBC number, structure, or function include:

anemia: a deficit in healthy RBCs or hemoglobin that impairs oxygen delivery. Iron deficiency anemia, megaloblastic anemia, and aplastic anemia are among common forms; symptoms often include fatigue and pallor.
polycythemia: an excess of RBCs that can increase blood viscosity and risk of thrombosis.
sickle cell disease: a hemoglobin mutation that causes RBCs to assume a rigid, sickle-like shape under low oxygen, with consequences for blood flow and organ function.
thalassemia: inherited disorders affecting hemoglobin synthesis, leading to imbalanced globin chains and RBC destruction.
hemolysis and related conditions: accelerated RBC destruction can lead to anemia and the need for transfusion or iron handling.

These topics intersect with clinical practice, including decisions about blood transfusion (using packed red blood cells), donor selection, and transfusion safety. The management of RBC-related disorders often involves balancing costs, access to care, and the burden of treatment, which are typical policy concerns in healthcare systems that emphasize both efficiency and universal coverage. See iron deficiency and sickle cell disease for more detail on specific conditions.

Therapeutics, transfusion, and storage

Medical use of RBCs frequently involves transfusion, especially in acute blood loss, surgery, and certain anemias. Blood typing and compatibility testing minimize immune reactions, while storage techniques preserve RBC viability for weeks under controlled conditions. Alternative strategies include erythropoiesis-stimulating approaches and, in experimental settings, RBC substitutes or transfusion alternatives. See blood transfusion and blood donation for related topics, and storage lesion for preservation concerns.

The interface of RBC science with policy involves debates about donor recruitment, compensation, and the efficiency of public vs. private blood services. Proponents of market-based approaches argue for competition to drive innovation and reduce waste, while supporters of universal public provision emphasize equity, safety, and rapid response in emergencies. These debates influence how societies structure blood collection, testing, and distribution, even as the underlying biology of RBCs remains a constant foundation for medicine.